1. Field of the Invention
This invention relates to a magnet roll for use in a developing device or a cleaning device in an image forming apparatus such as an electrophotographic copying apparatus or a facsimile apparatus or in a recording apparatus.
2. Description of the Prior Art
Developing devices of electrophotographic apparatus which use dry developer usually utilize the magnetic force of a magnet to effect conveyance of the developer to a development station and the actual development.
FIG. 1 of the accompanying drawings shows an example of the conventional developing device. An electrostatic latent image formed on the surface of a photosensitive medium 1 is developed by one-component magnetic toner 4 on a non-magnetic sleeve 3 containing a magnet roll 2 therein. The magnetic toner 4 is contained in a hopper 5 and is applied onto the sleeve 3 by a magnetic blade 6 disposed in opposed relationship with a magnetic pole N1 and is conveyed to a developing station by the rotation of the sleeve 3 in the direction of the arrow. Designated by 7 is a power source for applying a developing bias between the sleeve 3 and an electrode 8 disposed on the back of the photosensitive medium 1.
As the magnet roll 2 used for such purpose, use has heretofore been made of an isotropic or anisotropic ferrite sintered magnet. However, such a sintered magnet has required a special molding apparatus in the manufacture thereof and has been liable to break off and has greatly varied in dimensions during the sintering, and this has led to the necessity of grinding the very hard surface of the magnet after sintered. Also, the sintering reaction takes place at high temperatures, which has led to the disadvantage that a great deal of energy is consumed and the disadvantage that the magnet is very heavy.
To overcome these disadvantages, it has been proposed to unitarily shape a rubber or plastic magnet into the form of a roll and secure the roll to a supporting shaft or adhesively secure a plurality of magnet pieces to a supporting shaft and use the assembly as a magnet roll, but these proposals have not yet been put into practice because the magnetic force obtained is weak. Further, to obtain a rubber or plastic magnet having a strong magnetic force, it has been attempted to render such magnet anisotropic and in fact, it has become possible to obtain a magnet having a maximum energy product of the order of 10.sup.6 Gauss.Oe, but this magnet is still somewhat insufficient in magnetic force to be used as the magnet roll in the developing device of an electrophotographic apparatus or the like.
Further, in the case of a magnet roll comprising a magnet secured to a supporting shaft as described above, if the supporting shaft is a round bar or the like having a circular cross-sectional shape, the supporting shaft used must necessarily have a diameter greater than a certain value so that it may have a sufficient strength. Also, particularly in the case of a small-diameter magnet roll it is not possible to secure sufficient thickness of the magnet and it is low in permeance. Therefore, the magnetic force thereof has been weak and attempts to make the diameter of the magnet roller smaller have been hampered.
Where a magnet roll is to be formed by adhesively securing a plurality of axially extending magnet pieces a supporting shaft, if the supporting shaft is a round bar, the adhesively secured surfaces of the magnet pieces are curved surfaces and therefore, sufficient adhesion strength cannot be obtained and in particular, it has been difficult for a flexible magnet such as a rubber or plastic magnet to be adhesively secured to the supporting shaft with good accuracy.
Also, where a magnet roll is used in the developing device of an electrophotographic apparatus, particularly, in the developing device of the type as shown in FIG. 1 wherein the magnet roll is fixed and a non-magnetic sleeve is provided outside thereof and development is effected by rotation of the sleeve, it is important to properly dispose the magnetic poles of the magnet roll relative to the developing device. For this purpose, the end surfaces of the supporting shaft has heretofore been partly cut away as by milling. However, such milling has required a great deal of labor and it has been difficult to accurately determine the positions of the magnetic poles relative to the cutaway surfaces.